deduced that the single unpaired electron residesprimarily on the dioxygen moiety.104a,105From other experiments107it is appar-ent that net transferofelectron density from the metal onto the dioxygen varies

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2064 / BIOLOGICAL AND SYNTHETIC DIOXYGEN CARRIERSconsiderably, from aboutO.le-to about0.8e-.For example, itisfound for agivenCOIlSchiff base, Co(bzacen), that the redox potentialofthe cobalt-Schiff-base center LCo, measured by cyclic voltammetry,E1/2,B2LCo11I+e - () B2LCo"B=substituted pyridine(4.39)isa linear functionoflogK(02)as the axial base Bisvaried. The more easilytheCOIlcenter may be oxidized, the higheristheO2affinity,103asillustrated inFigure 4.18A. The dioxygen affinity also increasesasthe basicityofthe axialnitrogenous ligand increases.104aThis effectisillustrated in Figure 4.18B. Be-causeofdiffering steric requirements, dimethylformamide (DMF) , substitutedimidazole, and piperidine (pip) ligands do not fall on the correlation defined bythe seriesofsubstituted pyridine species. Note the synergistic natureofdioxy-gen binding: in general, the more electron density thatispumped onto the metalby the axial base, the more electron densityisavailable for donation into the7T*orbitalsofthe dioxygen ligand. El/2and logK(02)are also correlated, al-though more weakly, for a numberofhemoglobins (Figure 4.18C).108 Here theporphyrin and axial base remain constant, but presumably the surroundingsofthe heme group and O2binding site vary in a manner that is less well-definedthan in the model systemsofFigure 4.18A and B. Notwithstanding these var-iousperturbationstothemetalcenter,the0-0stretchoccursatabout1140 cm-1,placing all1:1 cobalt and iron-dioxygen complexesofnitrogenousand other hard ligands into the superoxo class.*Cobalt(II) porphyrins and their adducts with diamagnetic molecules invari-ably have spin S=i.(See Figure 4.16, but add one electron.) Thus the struc-tural changes are less pronounced than for corresponding iron(II) systems.110,111From the similarities in geometries and differences in electronic structures be-tween cobalt- substituted and native hemoglobins and their models, many in-sights have been gained about the factors that determine oxygen affinityaswell*Because the0-0stretch maybecoupled with other ligand modes,109its value should notbeused toestimate superoxo character, although in a seriesoffL-superoxo and fL-peroxo complexesofcarefully con-trolled stereochemistry, small changes in1'(0-0)have been correlated with thepKaofthe suiteofligands66Figure4,18(facing page)Linear free-energy relationships: (A) Correlationofthe O2affinity at21°CofCo(bzacen)Lwith theCOllI;;:::::COilcyclic voltammetric waveofCo(bzacen)L2species,I04a (B) Correlationofligand affinity withpKa-Squares(0)pertain to the bindingofL to Co(PPIX) at23°e.Circles(0)pertain to the O2affinity at - 45°CofCo(PPIX)L species, Filled shapes pertain to substi-tuted pyridines; the least-squares lines shown are calculated from these data only,104a(C) Corre-

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